Pre-1996 model year 6.5L diesels call for a standard ethylene glycol (such as ACDelco 10-5034) based engine coolant. This is your typical, most widely available solution and is generally green in color. 1996 and newer engines received GM's then new Dex-Cool engine coolant, a silicate free ELC (extended life coolant) that is orange in color. GM suggests that Dex-Cool (GM 12346290) has a service life of up to 5 years/150,000 miles. The service life of a standard green fluid varies considerably. Coolant types cannot be mixed, but by performing a thorough flush of the system you may swap coolant types. Simply draining the old coolant and refilling with new is not sufficient if you plan on converting coolant types.

Engine Coolant Selection & Dex-Cool Controversy

Many owners convert from Dex-Cool ELC to a traditional (generally green) ethylene glycol coolant. Complaints surrounding Dex-Cool engine coolant include premature water pump failure, clogged radiators, frequent overheating (sticking thermostats), and the deterioration of various seals. A number of lawsuits filed against GM allege that the engine coolant is responsible for problems in a variety of cars and trucks that arrived from the factory with Dex-Cool coolant.

Moral of the story, Dex-Cool is somewhat of a controversial product but if you would like to convert coolant types the procedures below will outline the process. If you're not interested in changing coolant types, the procedures below will still guide you through a typical cooling system service on a 6.5L GM diesel. We've chosen to use the ethylene glycol based Shell Rotella Ultra ELC, in part do to its long service life (advertised 600,000 miles). Such long service lives are typical in the medium and heavy duty diesel markets, and these extended life coolants are entirely compatible with engines such as the 6.5L. Another coolant that we can recommend is Fleetguard CC36073 ELC. Fleetguard is a Cummins' service brand and we've had nothing but positive experiences using this a variety of applications.

If you decide to switch from Dex-Cool to a traditional engine coolant, you are highly advised to use a cooling system flush/cleaning solution. If you are not changing coolant types, a chemical flush is not necessary but recommended nonetheless. We use Fleetguard Restore; it is designed to remove silicate gel and traces of fuel/oil in the cooling system. We tend to avoid cleaning/flushing solutions that remove rust, corrosion, etc as these particulates can get lodged in various places, including oil coolers. Alternatives include GM's ACDelco Fast Flush p/n 10953114.

In addition to changing the engine coolant, the upper radiator hose, lower radiator hose, and thermostat should be replaced (at minimum) during a cooling system service. If changing fluid types, the replacement of these items is required. The steps for replacing these items are included in the procedures below.

The following procedures require that you use a coolant concentrate. This process is NOT compatible with a premixed/diluted engine coolant. Regardless of what coolant you use, it must be a concentrate.

6.5L GM Diesel Cooling System Service Procedures

• Locate the drain valve on the driver side of the radiator. It does not sit below the bottom of the radiator and is easy to reach once you locate it. This is a particular messy drain system as there is no feasible way to attach a hose (in our experiences, but there are different types of radiators) to the valve and it will therefore run down the frame, leaf spring, etc. It may require using two separate drain pans in order to eliminate/reduce spillage.

• Drain the radiator into an appropriate container that has a minimum 4 gallon (16 quart) capacity. Do not use a drain pan that has previously had oil in it, as certain recycling centers will not accept engine coolant that has been contaminated.

• Remove the lower radiator hose at the radiator itself and allow to drain completely, then reinstall the hose.

• Now the flush process begins; it's very repetitive and we'll do our best to explain the reasoning behind these procedures. We prefer not to drain the engine block as the plugs can be extremely temperamental and the last thing we want is to strip threads in the engine block. The process outlined below is more time consuming but achieves the same end result.

• Fill the cooling system with distilled water through the overflow reservoir until the system has been refilled to its normal capacity.

• Start the engine and allow the cooling system to reach operating temperature (you will want to take it for a short drive). Allow the engine to cool, then drain the radiator.

Note - with each radiator drain we are only removing a portion of the total system capacity, thus it takes many flush cycles before the old engine coolant is removed completely (for all intents and purposes).

• Refill the radiator once more with distilled water, allow engine to reach operating temperature, then drain once the engine has cooled.

• Add your cooling system cleaner/flush agent per the manufacturer's instructions, then completely fill the cooling system with distilled water. Operate the vehicle per the cleaner's instructions. In our case, Fleetguard Restore needs to be run through the engine for a minimum of 60 minutes and a maximum 90 minutes once the engine reaches operating temperature.

• Allow the engine to cool then drain the radiator.

• Complete a minimum of 3 more flush cycles; drain, refill, drive, repeat. If after 3 cycles the water in the cooling system appears milky, foggy, or has any color to it, continue the flush process until it drains clean and clear. It ultimately took us 7 drain/refill cycles to flush the system.

• On your final drain cycle, the water should drain clear and with no trace of engine coolant or flushing agents. Leave the radiator empty at this point, then proceed with the steps below.

• Remove the upper radiator hose (connects thermostat housing to top of radiator).

• Remove the thermostat housing with a 16 mm socket. Be gentle with the stud on the driver side of the housing that mounts the fuel bleed valve.

• Remove and discard the old thermostat.

• Stuff a lint free rag into the lower portion of the thermostat housing and thoroughly clean the gasket mating surface. Repeat this for the upper portion of the thermostat housing; both mating surfaces must be completely clean before reinstalling.

• Install the new thermostat (take note of its orientation, do not install upside down) and thermostat gasket. If you are using the ACDelco gasket (recommended, ACDelco 12G11) you must remove the white plastic backing prior to installation. One side of this gasket features an adhesive and it will stick to the lower portion of the thermostat housing.

• Reinstall the upper thermostat housing and snug down the bolts. Do not overtighten, aluminum threads are easily stripped. Applying anti-seize to the bolt threads is recommended.

• Install the new upper radiator hose. It is ideal to use new hose clamps.

• Remove
and replace the lower radiator hose (connects water pump to bottom of radiator). It has an intermediate hose that tees into the radiator hose. This hose can be difficult to remove, so we suggest tracing it back to the coolant reservoir and disconnecting it. Then, remove the lower radiator hose and reservoir hose as one assembly.

It is much easier to remove the small hose this way. It is a tapered hose, so it cannot be replaced by standard heater hose if it is damaged.

• Replace the cap on the overflow reservoir/tank. Check the condition of the reservoir tank hoses and replace as necessary.

• Once the new thermostat and hoses have been installed/secured, fill the cooling system with coolant. You'll need to add 1/2 your total cooling system capacity. Refer to your owners manual for total cooling system capacity; it varies by model year and optional equipment. The 1995 GMC used in this tutorial has a 5.875 (23.5 quart) cooling system capacity. Since we want to achieve a 50/50 anti-freeze, distilled water ratio we are going to add 2.9 gallons (1/2 total capacity) of concentrated anti-freeze. Recall that whatever currently remains in our system at this point is pure water, for all intents and purposes. You must use concentrated coolant or your ratio will be incorrect.

• Open the bleed screw at the top of the thermostat housing before adding engine coolant.

• Once the necessary amount of engine coolant has been added to the cooling system, fill the reservoir with distilled water to the "max fill" indicator on the side of the tank. If the system is slightly overfilled at this point, don't be alarmed; it still needs to be purged of air.

Note - If at any time coolant begins to flow out of the bleed valve orifice, close it.

• With the overflow reservoir cap installed, start the engine.

• Watch the bleed valve on the thermostat housing and close it once a steady stream of coolant flows out of the orifice. It is sometimes useful to close the valve when it first begins to weep then take the truck for a quick test drive and crack the valve open again. Repeat until you witness a steady stream of fluid when the valve is opened. Close the valve once the system is purged.

• Allow engine to cool and verify coolant level. If low, add distilled water - do NOT add any additional anti-freeze beyond this point.

• You may need to repeat the bleed procedure several times in order to completely purge the system of air.

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